This invention relates to an improved support paper for photographic coatings containing a silver salt or salts.
There are known support papers for photographic coatings which are internally sized in the conventional manner with resin soaps, fatty acid soaps or fatty acid anhydrides, and as the sizing agent is precipitated with acid substances, for example aluminium sulphate, aluminium chloride, formic acid or the like, they are classified as so-called acid papers. Such papers have been used for decades either directly or, after coating with pigmented and/or water-repelling coatings, as supports for photographic coatings. The photographic coatings can be either those for black-and-white or those for colour photography.
For some years, furthermore, photographic paper supports have been known, which contain a neutral internal sizing. This so-called "neutral sizing" is based preferably upon the use of a dispersion of an alkyl ketene dimer, which is precipitated by means of a cationic substance, such as a polyamine and/or polyamide resin, onto the cellulose fibres and renders these hydrophobic. This type of internal sizing of paper is described, for example, in W. German Patent Application No. 2 710 061. Such neutrally sized papers are coated, like the conventional acid papers, for example with synthetic resins and are utilized in this form as supports for photographic coatings. This is described in W. German Patent Application No. 2 641 266.
Photographic papers not only have to be sized but must also be moisture resistant. Otherwise, they could lose strength and cohesiveness in photographic process solutions. Papers which are coated to render them waterproof on the surface, but are not internally water-resistant, split after treatment in photographic baths at their cut edges, sometimes by several mm. It is therefore essential, in addition to the sizing, to render them internally waterproof.
In the case of acid sized papers, for achieving the necessary internal waterproofness, condensed formaldehyde-releasing resins are used. Such resins are preferably melamine formaldehyde or urea formaldehyde resins in precondensed form. Optimum waterproofness of this photographic paper is achieved, with these resins, only with an acid pH of 5 or less than 5.
A disadvantage of the conventional acid papers which are rendered waterproof with resins containing formaldehyde, is that when stored, formaldehyde is released from formaldehyde-containing substances. Released formaldehyde influences the photographic coatings. The state of hardening of the coatings is modified, the tone of black-and-white pictures undergoes changes, and due to the reaction with the couplers in colour photographic coatings, undesired changes in colour are produced.
A disadvantage of acid-sized papers is, moreover, the low pH value of these papers. The sensitivity of photographic silver salt coatings decreases with decreasing pH, whereas photography in general aims at an increase in sensitivity. Finally, a further disadvantage is the low resistance of the acid sizing to modern alkaline developer solutions containing an organic solvent, e.g. benzyl alcohol. Consequently, the neutrally sized papers have come increasingly into favour. Neutrally sized papers require different waterproofing agents. Suitable waterproofing agents, which have become readily available since the end of the nineteen-sixties, are those which contain epoxy groups. In particular, polyamide or polyamine epichlorhydrin resins have proven effective as waterproofing agents in the neutral to slightly alkaline pH range.
Photographic papers neutrally waterproofed in this manner are, however, subject to defects which limit their serviceability. A major disadvantage of the waterproofed neutrally sized papers lies in the fact that all waterproofing agents manufactured from condensation products of epichlorhydrin with polyamines, polyimines and/or polyamides produce haze or ghosting in the photographic coatings which rest on the paper. This haze, even in papers which contain only 0.3 to 2% by wt. of such waterproofing agents, is still measurable in photographic coatings in contact therewith. Also, synthetic resin coatings situated between the paper and the photographic coating only temporarily protect the coating against the effect of these paper constituents which produce haze. In particular, highly sensitive black-and-white emulsions and coatings for diffusion transfer processes are affected by this haze.